Change Array.Get() to return Value instead of Storable

This commit changes Array.Get() to return Value instead of Storable.

Currently, Array.Get() returns Storable and client converts
returned Storable to Value.  However, it only makes sense to
return Storable if client needs to remove register (slab) by
StorageID (StorageIDStorable).

Get() should only provide value without possibility of client
manipulating the underlying storable.

array.go

@@ -1995,9 +1995,18 @@ func NewArrayWithRootID(storage SlabStorage, rootID StorageID) (*Array, error) {
 	}, nil
 }
 
-func (a *Array) Get(i uint64) (Storable, error) {
-	// Don't need to wrap error as external error because err is already categorized by ArraySlab.Get().
-	return a.root.Get(a.Storage, i)
+func (a *Array) Get(i uint64) (Value, error) {
+	storable, err := a.root.Get(a.Storage, i)
+	if err != nil {
+		// Don't need to wrap error as external error because err is already categorized by ArraySlab.Get().
+		return nil, err
+	}
+	v, err := storable.StoredValue(a.Storage)
+	if err != nil {
+		// Wrap err as external error (if needed) because err is returned by Storable interface.
+		return nil, wrapErrorfAsExternalErrorIfNeeded(err, "failed to get storable's stored value")
+	}
+	return v, nil
 }
 
 func (a *Array) Set(index uint64, value Value) (Storable, error) {

array_bench_test.go

@@ -25,7 +25,8 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-var noop Storable
+var noopValue Value
+var noopStorable Storable
 
 func BenchmarkArrayGet100x(b *testing.B) {
 	benchmarks := []struct {
@@ -223,18 +224,18 @@ func benchmarkArrayGet(b *testing.B, initialArraySize, numberOfOps int) {
 
 	array := setupArray(b, r, storage, initialArraySize)
 
-	var storable Storable
+	var value Value
 
 	b.StartTimer()
 
 	for i := 0; i < b.N; i++ {
 		for i := 0; i < numberOfOps; i++ {
 			index := r.Intn(int(array.Count()))
-			storable, _ = array.Get(uint64(index))
+			value, _ = array.Get(uint64(index))
 		}
 	}
 
-	noop = storable
+	noopValue = value
 }
 
 func benchmarkArrayInsert(b *testing.B, initialArraySize, numberOfOps int) {
@@ -307,7 +308,7 @@ func benchmarkArrayRemoveAll(b *testing.B, initialArraySize int) {
 		}
 	}
 
-	noop = storable
+	noopStorable = storable
 }
 
 func benchmarkArrayPopIterate(b *testing.B, initialArraySize int) {
@@ -336,7 +337,7 @@ func benchmarkArrayPopIterate(b *testing.B, initialArraySize int) {
 		}
 	}
 
-	noop = storable
+	noopStorable = storable
 }
 
 func benchmarkNewArrayFromAppend(b *testing.B, initialArraySize int) {

array_test.go

@@ -57,10 +57,7 @@ func verifyArray(
 
 	// Verify array elements
 	for i, v := range values {
-		s, err := array.Get(uint64(i))
-		require.NoError(t, err)
-
-		e, err := s.StoredValue(array.Storage)
+		e, err := array.Get(uint64(i))
 		require.NoError(t, err)
 
 		valueEqual(t, typeInfoComparator, v, e)
@@ -153,8 +150,8 @@ func TestArrayAppendAndGet(t *testing.T) {
 		require.NoError(t, err)
 	}
 
-	storable, err := array.Get(array.Count())
-	require.Nil(t, storable)
+	e, err := array.Get(array.Count())
+	require.Nil(t, e)
 	require.Equal(t, 1, errorCategorizationCount(err))
 
 	var userError *UserError

cmd/stress/array.go

@@ -417,14 +417,10 @@ func checkArrayDataLoss(array *atree.Array, values []atree.Value) error {
 
 	// Check every element
 	for i, v := range values {
-		storable, err := array.Get(uint64(i))
+		convertedValue, err := array.Get(uint64(i))
 		if err != nil {
 			return fmt.Errorf("failed to get element at %d: %w", i, err)
 		}
-		convertedValue, err := storable.StoredValue(array.Storage)
-		if err != nil {
-			return fmt.Errorf("failed to convert storable to value at %d: %w", i, err)
-		}
 		err = valueEqual(v, convertedValue)
 		if err != nil {
 			return fmt.Errorf("failed to compare %s and %s: %w", v, convertedValue, err)